1. Field of the Invention
This invention relates to styrene and acrylic free radical polymerization processes employing cobalt(II) chelates of vicinal iminohydroxyimino- and dihydroxyiminoalkanes, N,N'-bis(salicylidene)-ethylenediamines and porphyrins as catalytic chain transfer agents for controlling the molecular weight of the styrene and acrylic homopolymers and copolymers produced.
2. Background
In any polymerization process it is necessary to be able to control the molecular weight of the polymer produced so that it may be fitted to a particular use or need. For example, in unperturbed polymerization systems which fundamentally tend to produce high molecular weight polymers, it may be desirable or necessary to limit the molecular weight of the polymers produced, and this must be done in a predictable and controllable fashion. Such molecular weight limitation may be desirable or necessary in the preparation of polymer solutions for use in paints and finishes which require high solids contents to assure reduced solvent emission during application, and yet which require low viscosity to facilitate ready application.
In free radical polymerizations there are several conventional means of effecting such molecular weight limitation, but all have notable disadvantages. These include:
(1) The use of a high initiator/monomer ratio, but this is costly in terms of initiator consumption.
(2) Polymerizing at high temperatures, for example, about 150.degree. C., which is undesirably energy intensive.
(3) Adding stoichiometric amounts of thiol chain transfer agents to the polymerizing system, but the attendant incorporation of sulfur-containing agents into the polymer renders it less durable than is desired.
Catalytic chain transfer to the monomer as a means of controlling molecular weight in the radical polymerization of methyl methacrylate and styrene in the presence of cobalt(II) porphyrin complexes is known in the art. N.S. Enikolopyan et al., J. Polym. Sci., Polym. Chem. Ed., Vol. 19, 879 (1981), describe the kinetics and the molecular weight control achieved in the free radical polymerization of methyl methacrylate in the presence of a cobalt complex of hematoporphyrin tetramethyl ether. This use of this cobalt complex is also discussed by B. R. Smirnov et al. in Vysokomol. soyed., A23, No. 5, 1042 (1981) and by B. R. Smirnov et al. in Dokl. Akad. Naud SSSR, 253, 891 (1980). In a similar study, B. R. Smirnov et al., Dokl. Akad. Nauk SSSR, 254, 127 (1980), describe studies carried out with hematoporphyrin tetramethyl ester. The authors conclude that only the combination of cobalt with a tetrapyrrole porphyrin ligand apparently permits the realization of catalysis of chain transfer to the monomer, and that it has been ruled out that analogous phenomena will be discovered during the investigation of complexes similar in spatial and electronic structure to the porphyrins, such as phthalocyanines, corrins, cobaloximes, etc. B. R. Smirnov et al., Vysokomol. Soyed., A23, No. 11, 2588 (1981), describe the catalytic chain transfer observed in the radical polymerization of styrene in the presence of a cobalt complex of hematoporphyrin IX tetramethyl ester.
U.S. Pat. No. 4,526,945, issued to Carlson, discloses a process comprising polymerizing monomer, especially including methacrylate monomer, in the presence of a azo catalyst and between 0.0001% and 0.01% of Cobalt(II) dimethylglyoxime pyridine or similar Cobalt(II) complexes to produce low molecular weight polymer or copolymer.
D. E. Pashchenko et al., Dokl. Akad. Nauk SSSR, 265, 889 (1982), describe chain transfer studies with cobalt porphyrins in the polymerization of methyl methacrylate. Regarding this paper, it is not understood what is meant by "cobalt complexes of porphyrins and cobaloximes" since there is no further mention of "cobalozimes" in the English language version of the paper; moreover, the term "cobalozimes" does not appear in the original Russian test, but rather the term "cobalamines", which are vitamin B12-related structures similar to the porphyrin structures disclosed in this paper and in the other publications cited above.
Although the use of the porphyrin complexes circumvents many of the problems associated with the aforesaid conventional commercial processes, the complexes impart too much color to the final product, rendering it useless or less desirable in many applications, such as in certain paints and finishes. Moreover, the cost of the porphyrin complex is rather high.
A. F. Burczyk et al., J. Polym. Sci., Polym. Chem. Ed., Vol. 22, 3255 (1984), disclose that cobaloximes, that is, bisdimethylgoloximatocobalt complexes, are often used as analogs of cobalt porphyrins in research studies, and they further disclose the use of cobalozime, synthesized from Co(II) acetate and dimethylglyoxime, as a relatively cheap chain transfer agent in the free radical polymerization of methyl methacrylate. The cobalozime of Burczyk et al. is shown on page 3256 as being of the formula ##STR1## wherein B is a coordinating base ligand, such as triphenylphosphine. Stucture Va which constitutes a part of the invention which is disclosed and claimed herein reads on the cobalozime of Burczyk et al. Similar disclosures are made by A. F. Burczyk in a thesis to the University of Waterloo, Waterloo, Ontario, 1984.
U.S. Pat. No. 3,262,995, issued to Huff et al., discloses preparation of block polymers of acrylonitrile and .alpha.-substituted acrylonitriles and vinyl aromatic monomers such as styrene. The vinyl aromatic monomers are polymerized by a free-radical mechanism in the presence of a phosphine as a chain transfer agent to obtain a polymer of the vinyl aromatic compound containing phosphorus in the polymer chains. U.S. Pat. No. 3,846,383, issued to Uyama et al., discloses a method for producing olefin-unsaturated acid copolymer wherein an olefin and an unsaturated acid anhydride are polymerized in the presence of a radical type polymerization catalyst. An effective amount of a molecular weight lowering agent is incorporated into the polymerization reaction mixture, whereby molecular weight of the copolymer is lowered.
U.S. Pat. No. 3,860,568, issued to Chabert et al., discloses a process for polymerizing olefinic monomer by contacting the monomer with a polymerization initiator having three components. The initiator comprises, in combination, (1) an oxidizing agent, (2) a chelate compound of a metal of Group IB, IIB, IVB, VB, VIB, VIIB, IIIA, IVA, VA, and VIII of the Periodic Table with at least one bidentate ligand, and (3) an electron donor in an amount at most equal to that required to complex completely the metal of the chelate compound.
It is an object of this invention to provide a process which uses cobalt(II) chelates of vicinal iminohydroxyimino- and dihydroxyiminoalkanes, N,N'-bis(salicylidene)ethylenediamines and porphyrins which operate as highly efficient catalytic chain transfer agents for controlling styrene and acrylic homopolymer and copolymer molecular weights, some of which impart very little, if any, color to the final products, making them more useful in many applications, such as in paints and finishes. A further object is to provide such a process which uses, in conjunction with the chelate, a suitable Lewis base which will be defined hereinafter, which use offers a further means of controlling molecular weight. Another object is to provide such a process which proceeds at modest temperatures with limited amounts of initiator and in the absence of stoichiometrically-reacting chain transfer agents. These and other objects will become apparent hereinafter.
Some of the above objects have already been provided and are disclosed in commonly-assigned, United States patent application Ser. No. 707,367 which was filed Mar. 1, 1985, with the inventor herein being a joint inventor therein. More particularly, the application discloses and claims an improved free radical polymerization of acrylic and/or styrene monomers wherein molecular weight control is effected by means of a chain transfer agent, the improvement characterized in that the chain transfer agent is a cobalt chelate of the formula ##STR2## wherein each R, independently, is phenyl or C.sub.1 to C.sub.12 alkyl wherein each .alpha.-carbon atom contains two hydrogen atoms, or R and R on adjacent carbon atoms, taken together, id C.sub.5 to C.sub.8 cycloalkylene, unsubstituted in the .alpha.-positions; R.sup.2 is H or C.sub.x H.sub.2x+1 wherein x is 1 to 12; each R.sup.4 is H or both R.sup.4 groups taken together is --O--Z--O--; R.sup.7 is O or NH; n is 2 or 3; Z is H, BF.sub.2, BCl.sub.2, BBr.sub.2 or BR.sub.2.sup.2 ; X.sup.- is NO.sub.3 hu -, Cl.sup.-, Br.sup.-, I.sup.-, BF.sub.4.sup.-, PF.sub.6.sup.-, SbF.sub.6.sup.- or R.sup.1 COO.sup.- wherein R.sup.1 is C.sub.1 to C.sub.12 alkyl; and L is ##STR3## H.sub.2 O or (C.sub.6 H.sub.5).sub.3 P; provided, however, each of either or both aromatic rings in VIII optionally is substituted with a benzo group and optionally contains up to four substituents.
G. N. Schrauzer, Inorg. Syn., 11, 64 (1968), describes the preparation of diaqua bis(2,3-dihydroxyiminobutanato)-Co(II), see Example 1 herein.